1. Field of the Invention
The present invention relates to methods for preparing aqueous colored pigment dispersions. Inkjet ink compositions are also disclosed.
2. Description of the Related Art
In general, pigments alone are not readily dispersible in liquid vehicles. A variety of techniques have been developed which can provide stable pigment dispersions. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants for an aqueous medium include water-soluble polymers and surfactants.
Colored pigment dispersions have been prepared using dispersants having a structural unit similar if not identical to the chemical unit of the colored pigment. For example, GB2356866 discloses a bis-azo compound derived from diazotized (carboxy/sulfo)-anilines and 1,4-bis(acetoacetamido)phenylenes and their use in pigment and ink compositions. These materials were found to be particularly useful in combination with yellow or orange pigments having nearly the identical structure. GB2356634 describes very similar mono-azo compounds for the same use. GB2364322 describes N-(sulfophenyl)-alpha-(2-methoxy-4-nitrophenylazo) acetacetamides for use with monoazo pigments, particularly yellow pigments.
These colored pigment dispersants are prepared by the addition of a diazonium salt (the diazo component) with a compound capable of reacting with a diazonium salt (the azo coupling component). Once prepared, the resulting dispersant is then added, under specified conditions, to the colored pigment in order to form a colored pigment dispersion.
Other methods of controlling the dispersibility of colored pigments are also known. For example, International Patent Application No. WO00/26304 discloses the preparation of a crystal growth inhibitor that controls the amount of crystallinity developed during the process of preparing a colored pigment and prevents recrystallization of dispersed pigments. The crystal growth inhibitor is prepared using a process similar to the known process for preparing the pigment (for example, by the reaction of a diazonium salt with an azo coupling agent), and can also be prepared in situ during the preparation of the colored pigment. Also, in U.S. Pat. No. 6,451,103, water soluble pigment derivatives, which are pigments further substituted with an acidic functional group or salt, are used to prepare aqueous dispersions of that specific pigment. Inkjet inks are also disclosed.
Modified colored pigments have also been developed which provide ink compositions with improved properties, such as dispersibility. For example, U.S. Pat. No. 5,851,280 discloses methods for the attachment of organic groups onto pigments including, for example, attachment via a diazonium reaction wherein the organic group is part of the diazonium salt. The resulting surface-modified pigments can be used in a variety of applications, such as inks, inkjet inks, coatings, toners, plastics, rubbers, and the like.
PCT International Publication No. WO 01/51566 describes a method of making a modified pigment by reacting a first chemical group and a second chemical group to form a pigment having attached a third chemical group. The first chemical group includes at least one nucleophile and the second chemical group includes at least one electrophile, or vice versa. These pigments are used in ink compositions and, in particular, inkjet ink compositions.
While these efforts provide modified pigments with improved properties and, in particular, improved dispersibility, there remains a need for methods of preparing aqueous colored pigment dispersions and inkjet ink compositions.